In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are substantially solid or gelatinous at ambient temperature, but transition to liquid at an elevated temperature. The molten ink can then be ejected directly onto a recording substrate, such as paper, (referred to as direct printing or direct marking), or onto an intermediate imaging member, such a drum or belt, for subsequent transfer to a recording substrate (referred to as offset printing).
In both the direct and offset printing architecture, images may be formed on cut media sheets or a very long, i.e., continuous, web of media. When using melted phase change ink to form images on a media, whether it be cut sheet or continuous web, the ink may be fixed to the media by the application of pressure and/or heat to the ink on the media. For example, after melted phase change ink is placed on media, the media may be guided to a fixing assembly that includes an image side roller for contacting the freshly deposited ink and a pressure roller leveraged against the image side roller to provide a pressurized nip through which the media is fed. As the media is fed through the nip, the image side roller presses the deposited ink against the media to take what are essentially isolated droplets of ink and smear them out to make a continuous layer so that spaces between adjacent drops are filled and image solids become uniform. In addition to spreading the ink, pressing the ink against the media in this manner may also improve image permanence by increasing ink layer cohesion and/or increasing ink-to-media adhesion. The image side roller, the pressure roller or both may be configured to heat the ink and web to enhance the ability of the ink to spread onto and adhere to the media.
Phase change ink web printers may be configured to print images onto a single side of the web, referred to herein as simplex printing, or onto both sides of the web, referred to herein as duplex printing. Duplex web printers may have, as examples, a serial web printing architecture or a mobius web printing architecture. In one type of serial architecture, a web of substrate material is guided through a first side print station for deposition of melted phase change ink onto a first side of the web and then guided through a pressurized nip of a first fixing assembly for spreading the ink and fixing the ink to first side of the web. The web may then be flipped over, or inverted, and guided through a second side print station for deposition of melted phase change ink onto the second side of the web. The web, after having been printed on the second side, is then guided through a pressurized nip of a second fixing assembly for spreading the ink and fixing the ink on the second side of the web. In a mobius arrangement, two strands of the same media web are fed side by side through a print station that is configured to print onto both of the strands simultaneously. The first side of the web is facing toward the print station for one of the strands, and the second side of the web is facing toward the print station for the other strand. After the two strands have been printed onto at the print station, both strands are fed through the pressurized nip of a fixing assembly. The strand with the first side of the web facing toward the print station is guided to the entrance of the print station to serve as the second strand of the web and inverted along the way so that the second side of the web is facing toward the print station.
One difficulty faced in duplex printing is damage to the fixed ink on the first side of the media while fixing ink to the second side of the media. For example, when fixing the ink on the second side of the web, the fixed ink on the first side of the web is contacted by the pressure roller. Depending on the temperature and pressure in the nip, the fixed ink may offset to the pressure roller resulting in degradation of the images on the first side of the media. To prevent ink offset to the pressure roller while duplex printing on cut sheets, the pressure roller is allowed to scavenge some of the release agent from the image side roller during the intermittent contact with the image side roller between the passages of sheets. In web printing, however, the web acts as a constant barrier between the image side roller and the pressure roller of a fixing assembly.
To prevent ink offset to the pressure roller in duplex web printers, previously known duplex printers have decreased the temperature setpoints of the print process components relative to cut sheet printers so that the web is at a lower temperature when entering the pressurized nip of the fixing assembly. The lower temperature of the ink entering the nip decreases the malleability of the melted phase change ink and, therefore, reduces the propensity of the ink to offset to the pressure roller. The lower temperature of the ink entering the nip, however, necessitates an increase in the pressure in the nip in order to maintain the same degree of ink spread and corresponding image quality. The increased pressure requirements at the fixing nip in turn increases the wear on the image side roller and pressure roller which decreases the usable life of the rollers.